Linux production notes¶

Non-Uniform Memory Access (NUMA) hardware¶

Typically, a server motherboard has several sockets, each housing a physical CPU, connected to its own subset of DIMM and PCIe slots. Since access to another socket's RAM has to be mediated by that socket's CPU, such an architecture is called Non-UniformMemory Access, or NUMA. From the kernel's perspective, physical sockets are referred to as nodes.

You can see the kernel's view of your system's topology with numactl -H. When allocating memory, the kernel has to decide which node's free pages to use. This decision is governed by 'NUMA policies', which can be set by running q under numactl. Note that q itself is not aware of NUMA.

Since memory access is fastest within one node, if you know that your workload fits into one node's memory, you should just restrict the process to that node for maximum performance with the following to run q within node 0:

numactl -N 0 -m 0 q ...

By default, linux would try to allocate on the 'current' node (i.e. that on to which the CPU currently executing q belongs), however as the processes are moved by the scheduler freely across node boundaries, one ends up with pages allocated on multiple nodes, which leads to inconsistent performance.

If, however, the working set routinely and significantly exceeds a single node, you might prefer to allocate round-robin on all (or a subset of) nodes, e.g. for all nodes:

numactl -i all q ...

This ensures that the memory is distributed evenly, which results in lower but very predictable performance.

CPU affinity – Linux

See also the documentation for set_mempolicy(2) for a discussion on the various policies.

Huge Pages and Transparent Huge Pages (THP)¶

A number of customers have been impacted by bugs in the Linux kernel with respect to Transparent Huge Pages. These issues manifest themselves as process crashes, stalls at 100% CPU usage, and sporadic performance degradation.

Our recommendation for THP is similar to the recommendation for memory interleaving.

Linux kernel	THP
2.6 or earlier	disable
3.x or higher	enable

Other database vendors are also reporting similar issues with THP.

Note that changing Transparent Huge Pages isn’t possible via sysctl(8). Rather, it requires manually echoing settings into /sys/kernel at or after boot. In /etc/rc.local or by hand. To disable THP, do this:

if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
  echo never > /sys/kernel/mm/transparent_hugepage/enabled
fi

if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
  echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi

Some distributions may require a slightly different path, e.g:

echo never >/sys/kernel/mm/redhat_transparent_hugepage/enabled

Another possibility to configure this is via grub

transparent_hugepage=never

To enable THP for Linux kernel 3.x, do this:

if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
  echo always > /sys/kernel/mm/transparent_hugepage/enabled
fi

if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
  echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi

Q must be restarted to pick up the new setting.

Monitoring free disk space¶

In addition to monitoring free disk space for the usual partitions you write to, ensure you also monitor free space of /tmp on Unix, since q uses this area for capturing the output from system commands, such as system "ls".

Disk space for log files

It is essential to ensure there is sufficient disk space for tickerplant log files, as in the event of exhausting disk space, the logging mechanism may write a partial record, and then drop records, thereby leaving the log file in a corrupt state due to the partial record.

Back up the sym file¶

The sym file is found in the root of your HDB. It is the key to the default enums.

Regularly back up the sym file outside the HDB.

Compression¶

If you find that q is seg faulting (crashing) when accessing compressed files, try increasing the Linux kernel parameter vm.max_map_count. As root

sysctl vm.max_map_count=16777216

and/or make a suitable change for this parameter more permanent through /etc/sysctl.conf. As root

echo "vm.max_map_count = 16777216" | tee -a /etc/sysctl.conf
sysctl -p

You can check current settings with

more /proc/sys/vm/max_map_count

Assuming you are using 128-KB logical size blocks for your compressed files, a general guide is, at a minimum, set max_map_count to one map per 128 KB of memory, or 65530, whichever is higher.

If you are encountering a SIGBUS error, please check that the size of /dev/shm is large enough to accommodate the decompressed data. Typically, you should set the size of /dev/shm to be at least as large as a fully decompressed HDB partition.

Set ulimit to the higher of 4096 and 1024 plus the number of compressed columns which may be queried concurrently.

ulimit -n 4096

lz4 compression

Certain releases of lz4 do not function correctly within kdb+.

Notably, lz4-1.7.5 does not compress, and lz4-1.8.0 appears to hang the process.

Kdb+ requires at least lz4-r129. lz4-1.8.3 works. We recommend using the latest lz4 release available.

Timekeeping¶

Timekeeping on production servers is a complicated topic. These are just a few notes which can help.

If you are using any of local time functions .z.(TPNZD) q will use the localtime(3) system function to determine time offset from GMT. In some setups (GNU libc) this can cause excessive system calls to /etc/localtime.

Setting TZ environment helps this:

export TZ=America/New_York

or from q

setenv[`TZ;"Europe/London"]

One more way of getting excessive system calls when using .z.(pt…) is to have a slow clock source configured on your OS. Modern Linux distributions provide very low overhead functionality for getting current time. Use tsc clocksource to activate this.

echo tsc >/sys/devices/system/clocksource/clocksource0/current_clocksource
# list available clocksource on the system
cat /sys/devices/system/clocksource/clocksource*/available_clocksource

If you are using PTP for timekeeping, your PTP hardware vendor might provide their own implementation of time. Check that those utilize VDSO mechanism for exposing time to user space.